1. Field of the Invention
The present invention relates generally to a method for producing gas diffusion electrodes which include at least an electrically conductive catalyst support and a sheet-like structure produced by rolling a powder mixture. The powder mixtures generally contain at least one catalyst or a catalyst mixture and a binder.
2. Description of Related Art
DE-A 37 10 168 and EP-A 297 377 describe the production of gas diffusion electrodes by rolling dry powder mixtures containing a compound of the catalyst material, for example silver(I) oxide, and polytetrafluoroethylene (PTFE) as a binder. After the rolling of the powder mixture to give a sheet-like structure, the latter is applied to or rolled in a mechanical support which serves for supplying current to or conducting current away from the electrode. The mechanical support may be, for example, a metal net, nonwoven metal fabric or woven metal fabric or a carbon net, nonwoven carbon fabric or woven carbon fabric. The application of the sheet-like structure to the mechanical support can be effected, for example, by pressing or rolling.
Properties of sheet-like structures which are produced by rolling the powder mixtures are dependent on a multiplicity of parameters. In addition to the powder characteristics themselves, these parameters include, inter alia, the geometry and the surface characteristics of the rolls, the speed of the rolls, the precision of the bearings used, the accuracy of the bearings running on one hand, and the accuracy of the rolls on the other hand, the roll nip, and the clamping force of the rolls, to name a few. If, for example, the roll nip varies drastically during the rolling process, sheet-like structures whose thickness and density are not sufficiently homogeneous are obtained. If, on the other hand, the roll nip is kept substantially constant, for example, by using an excessively high clamping force, the electrodes exhibit, inter alia, very disproportionate voltage increases with increasing current density, as well as poor electrochemical activity during operation. Furthermore, excessively thick and mechanically unstable sheet-like structures generally cannot be used for further processing. Regarding the technical handling properties, such sheet-like structures generally should have a length of from 2 to 3 meters and a width of from 30 to 40 cm in order to be suitable for use as a gas diffusion electrode for electrochemical production of chlorine from aqueous solutions of alkali metal chloride.
For stability reasons, commercial rolls having a width of about 40 cm preferably have diameters of about 15 cm or more, depending on the maximum clamping forces to be applied. It was found that sheet-like structures which were produced with such rolls and were used as gas diffusion electrodes often did not have sufficient electrochemical activity. Particularly, this greatly increases, the operating costs of electrolysis cells and the like. The electrochemical activity is insufficient, for example, if the electrode has a cell voltage of over 2.5 V at a current density of 4 kA/m2.